Because the police (and many in society as a whole) were/are racist, misogynist and homophobic. This is why DEI initiatives had to be put in place - to try and mitigate the ingrained behaviours of the police.
The police believed the perpetrator in this case because for generations, the accusation of racism would have been accurate. If it wasn't for the generations of racist, misogynistic and homophobic behaviour, from the police and wider society, there would be no need for anyone to automatically believe an accusation of racism because of historic behaviour.
The only reason people are able to claim 'two tier policing' now, is due to appalling behaviours in the past. If policing had been fair, there would be no need for DEI. I have assumed, possibly erroneously, that you are a woman. If you are a woman, you should be grateful for DEI.
This case may not be about misogyny and homophobia specifically, but discrimination in the past has led us to where we are today.